Extreme ultra-violet (EUV) Lithography presents a unique challenge for computational lithographers, particularly within the realm of Optical Proximity Correction (OPC) software. While advances in model accuracy are naturally required with each new advanced node due to smaller feature sizes and tighter overlay requirements, EUV lithography adds additional complications not seen previously in deep ultra-violet (DUV) Lithography. Non-telecentric optics produce horizontal and vertical print differences on identical but rotated structures, reflective multilayer optics produce substantially higher aberration levels and stray light scattering (flare), and light reflections from the over-scanned “dark” regions of the mask produce halo exposures in neighboring fields, giving rise to the misnomer “black border modeling”. The modeling challenge is heightened since these effects additionally change through field position at a full chip/reticle scale.
An individual lithographic mask will typically contain many product dies (‘chips’) within a single field. FIG. 3 illustrates an example of multiple dies 310-360 placed on the reticle within the field. The pair of dies 310 and 320 share the same circuit design, and the pair of dies 350 and 360 share the same circuit design. In DUV lithography, OPC on any individual product die is performed independent of its position within the field, meaning OPC is field position independent. A product die could be placed on the left or the right, top or the bottom, without any change to the OPC it received. This had many advantages because identical product die could be corrected a single time, and placed multiple times within the field. From a modeling perspective, OPC models could be considered unchanged everywhere within the field: M(x,y)=M, where M is an arbitrary model (resist model, mask electro-magnetic field model, optical illuminating and imaging system model, etc.).
EUV lithography fundamentally alters this paradigm, requiring OPC to operate with knowledge of position in the field, and likewise understand how the underlying OPC models change depending on location. Under EUV lithography, the two product die placements 350 and 360 in FIG. 3 would both receive different OPC since they lie in different positions in the field. The same is true for the two die placements 310 and 320. Field position dependencies arise both from non-idealities in the EUV scanner (flare, aberrations), as well as intrinsic properties engineered into the EUV scanner designs (change of chief ray through slit). Piecewise position-dependent static input model strategies have been used to resolve field position dependencies. However, they produce critical dimension (CD) discontinuities, which leads to real wafer edge placement errors (EPEs).